12.95. As described in previous sections, design basis accidents are used to establish design requirements for plant safety systems. In the event of such an accident, the proper operation of such safety systems would prevent melting of the reactor core. However, as a result of the Three-Mile Island accident in 1979 (§12.179), in which partial core melting occurred, it be­came clear that studies were needed of accident scenarios that would lead to core degradation and the resulting possible transport of released fission products to the environment. The term severe accident has come to des­ignate such an accident for analysis purposes. Such studies are helpful in establishing confidence levels and identifying possible failure modes. An accompanying research program has provided supporting data.

12.96. Severe accident scenarios commonly used evolved from those developed in the Reactor Safety Study (RSS), which is often referred to as WASH-1400 [6]. Since the engineered safety features are provided to prevent fuel degradation, to have a severe accident we must not only have an initial “fault” such as a loss-of-coolant break but an additional failure of one or more of these engineered safety features. We will deal with the three representative sequences for PWRs that are often studied as well as two important BWR sequences. We are including accident sequence sym­bols that originated with the Reactor Safety Study (§12.208) only as a convenience to readers who may be familiar with them.